In my experience, which not include the TI (I sail an AI), monohull sailboats have better upwind performance than multihull sailboats.
I was a bit surprised that I did about the same speed as an Hobie Oasis from start to finish with a lot of tacking. Sometimes I was ahead of the Oasis, sometimes they were ahead of me. The Oasis pointed closer to the wind and they had no problem with the quite strong wind.
They (2 persons) used the Mirage Drive and so did I, combined with saling.
But as soon as we went sailing not so closed to wind, the AI left the Oasis behind. They had absolutely no chance to compete. And we were still going a little upwind.
As pointed out by all posts above, the Mirage Drive is a part of the concept and there is no "cheating", just the fastest boat from A to B with just wind & muscle.
Of course you can go upwind! But upwind performance is not the best sailing ability of an AI (and probably not a TI).
But I am still learning how to sail the AI with best possible speed and there are plenty more experienced AI/TI sailors on this forum.